[PATCH] Fix early debug regression with DW_AT_string_length (PR debug/71906)

[Richard Biener]

On August 9, 2016 4:36:24 PM GMT+02:00, Jakub Jelinek <jakub@redhat.com> wrote:
>On Mon, Aug 01, 2016 at 12:44:30PM +0200, Richard Biener wrote:
>> Note that reading the dwarf standard, it looks like it accepts a
>location
>> which means we could do an implicit location description using
>> DW_OP_stack_value which gives us access to arbitrary dwarf
>> expressions (and thus the possibility to handle it similar to VLAs).
>
>Sure.  But for the DW_AT_string_length, the issue is that with early
>debug,
>we want the attribute early, but locations within a function obviously
>can be provided late, after the function is compiled.
>
>Which is why the patch uses DW_OP_call4 as the holder of the
>information
>that the string length is provided by the value of some variable.
>In certain cases it can stay that way even during late debug, if the
>var
>disappears or has no location info etc., then DW_AT_string_length is
>dropped, or in other cases the location of the var is copied into the
>attribute etc.
>
>This is different from the VLA bounds in DW_AT_upper_bound etc., where
>it is allowed to just refer to a DIE that holds the value (so we can
>stick
>it into the attribute early and just supply the var's location later),
>for
>DW_AT_string_length it is not allowed.
>
>> But maybe I am missing something?  (now running into the issue
>> with LTO debug and gfortran.dg/save_5.f90 where during early debug
>> we emit a location that ends up refering to a symbol that might be
>> optimized away later - early debug cannot sanitize referenced symbols
>> via resolv_addr obviously).  Annotating the DIE late is also not
>> what I want to do as I'd need to pull in all type DIEs into the late
>CU
>> that way (well, at least selected ones, but I'm really trying to
>avoid
>> going down that route).
>
>In some cases like location of file scope vars, or this
>DW_AT_string_length,
>you really need to adjust late the debug info created early, there is
>no
>workaround for that.

I suppose the workaround is to fix/extend DWARF... (DW_AT_GNU_string_length that allows us to refer to another DIE?)

Richard.

>	Jakub